PRECAMBRIAN ROCKS THE CORE OF THE TETONS
The visitor who looks at the high, rugged peaks of the Teton Range is seeing rocks that record about seven-eighths of all geologic time. These Precambrian rocks are part of the very foundation of the continent and are therefore commonly referred to by geologists as basement rocks. In attempting to decipher their origin and history we peer backward through the dim mists of time, piecing together scattered clues to events that occurred billions of years ago, perhaps during the very birth of the North American Continent. To cite an oft-quoted example, it is as though we were attempting to read the history of an ancient and long-forgotten civilization from the scattered unnumbered pages of a torn manuscript, written in a language that we only partially understand.
Ancient gneisses and schists
The oldest Precambrian rocks in the Teton Range are layered gneisses and schists exposed over wide areas in the northern and southern parts of the range and as scattered isolated masses in the younger granite that forms the high peaks in the central parts. The layered gneisses may be seen easily along the trails in the lower parts of Indian Paintbrush and Death Canyons, and near Static Peak.
The layered gneisses are composed principally of quartz, feldspar, biotite (black mica), and hornblende (a very dark-green or black mineral commonly forming rodlike crystals). Distinct layers, a few inches to several feet thick, contain different proportions of these minerals and account for the banded appearance. Layers composed almost entirely of quartz and feldspar are light-gray or white, whereas darker gray layers contain higher proportions of biotite and hornblende.
Some layers are dark-green to black amphibolite, composed principally of hornblende but with a little feldspar and quartz. In many places the gneisses include layers of schist, a flaky rock, much of which is mica. At several places on the east slopes of Mount Moran thin layers of impure gray marble are found interleaved with the gneisses. West of Static Peak along the Alaska Basin Trail a heavy dark rock with large amounts of magnetite (strongly magnetic black iron oxide) occurs as layers in the gneiss.
In some places the gneiss contains dark-reddish crystals of garnet as much as an inch in diameter. Commonly the garnet crystals are surrounded by white "halos" which lack biotite or hornblende, probably because the constituents necessary to form these minerals were absorbed by the garnet crystals. In Death Canyon and on the slopes of Static Peak some layers of gray gneiss contain egg-shaped masses of magnetite as much as one-half inch in diameter (fig. 20). These masses are likewise surrounded by elliptical white halos and have the startling appearance of small eyes peering from the rock. Appropriately, this rock has been called the "bright-eyed" gneiss by Prof. Charles C. Bradley in his published study (Wyoming Geological Association, 1956) of this area.
What were the ancient rocks from which the gneisses of the Teton Range were formed? Most of the evidence has been obliterated but a few remaining clues enable us to draw some general conclusions. The banded appearance of many of the gneisses suggests that they were formed from sedimentary and volcanic rocks that accumulated on the sea floor near a chain of volcanic islandsperhaps somewhat similar to the modern Aleutians or the islands of Indonesia. When these deposits were buried deep in the earth's crust the chemical composition of some layers may have undergone radical changes. Other layers, however, still have compositions resembling those of younger rocks elsewhere whose origins are better known. For example, the layers of impure marble were probably once beds of sandy limestone, and the lighter colored gneiss may have been muddy sandstone, possibly containing volcanic ash. Some dark amphibolite layers could represent altered lava flows or beds of volcanic ash; others may have resulted from the addition of silica to muddy magnesium-rich limestone during metamorphism. The magnetite-rich gneiss probably was originally a sedimentary iron ore.
Minerals that were most easily altered at depth reacted with one another to form new minerals more "at home" under the high temperature and pressure in this environment just as the ingredients in a cake react when heated in an oven. Rocks formed by such processes are called metamorphic rocks; careful studies of the minerals that they contain suggest that the layered gneisses developed at temperatures as high as 1000°F at depths of 5 to 10 miles. Under these conditions the rocks must have behaved somewhat like soft taffy as is shown by layers that have been folded nearly double without being broken (fig. 21). Folds such as these range from fractions of an inch to thousands of feet across and are found in gneisses throughout the Teton Range. In a few places folds are superimposed in such a way as to indicate that the rocks were involved in several episodes of deformation in response to different sets of stress during metamorphism.
When did these gneisses form? Age determinations of minerals containing radioactive elements show that granite which was intruded into them after they were metamorphosed and folded is more than 2.5 billion years old. They must, therefore, be older than that. Thus, they probably are at least a billion years older than rocks containing the first faint traces of life on earth and 2 billion years older than the oldest rocks containing abundant fossils. How much older is not known, but the gneisses are certainly among the oldest rocks in North America and record some of the earliest events in the building of this continent.
Irregular bodies of granite gneiss are interleaved with the layered gneisses in the northern part of the Teton Range. The granite gneiss is relatively coarse grained, streaky gray or pink, and composed principally of quartz, feldspar, biotite, and hornblende. It differs from enclosing layered gneisses in its coarser texture, lack of layering, and more uniform appearance. The dark minerals (biotite and hornblende) are concentrated in thin discontinuous wisps that give the rock its streaky appearance.
The largest body of granite gneiss is exposed in a belt l to 2 miles wide and 10 miles long extending northeastward from near the head of Moran Canyon, across the upper part of Moose Basin, and into the lower reaches of Webb Canyon. This gneiss may have been formed from granite that invaded the ancient sedimentary and volcanic rocks before they were metamorphosed, or it may have been formed during metamorphism from some of the sediments and volcanics themselves.
At several places in Snowshoe, Waterfalls, and Colter Canyons the layered gneisses contain discontinuous masses a few tens or hundreds of feet in diameter of heavy dark-green or black serpentine. This rock is frequently called "soapstone" because the surface feels smooth and soapy to the touch. Indians carved bowls (fig. 22) from similar material obtained from the west side of the Tetons and from the Gros Ventre Mountains to the southeast. Pebbles of serpentine along streams draining the west side of the Tetons have been cut and polished for jewelry and sold as "Teton jade"; it is much softer and less lustrous than real jade. The serpentine was formed by metamorphism of dark-colored igneous rocks lacking quartz and feldspar.
Granite and pegmatite
Contrary to popular belief, granite (crystalline igneous rock composed principally of quartz and feldspar) forms only a part of the Teton Range. The Grand Teton (fig. 6) and most surrounding subsidiary peaks are sculptured from an irregular mass of granite exposed continuously along the backbone of the range from Buck Mountain northward toward upper Leigh Canyon. The rock is commonly fine grained, white or light-gray, and is largely composed of crystals of gray quartz and white feldspar about the size and texture of the grains in very coarse lump sugar. Flakes of black or dark-brown mica (biotite) and silvery white mica (muscovite) about the size of grains of pepper are scattered through the rock.
From the floor of Jackson Hole the granite cliffs and buttresses of the high peaks appear nearly white in contrast to the more somber grays and browns of surrounding gneisses and schists. These dark rocks are laced by a network of irregular light-colored granite dikes ranging in thickness from fractions of an inch to tens of feet (fig. 23).
The largest masses of granite contain abundant unoriented angular blocks and slabs of the older gneisses. These inclusions range from a few inches in diameter (fig. 24) to slabs hundreds of feet thick and thousands of feet long.
Dikes or irregular intrusions of pegmatite are found in almost every exposure of granite. Pegmatite contains the same minerals as granite but the individual mineral crystals are several inches or even as much as a foot in diameter.
Some pegmatites contain silvery plates or tabular crystals of muscovite mica as much as 6 inches across that can be split into transparent sheets with a pocket knife. Others have dark-brown biotite mica in crystals about the size and shape of the blade of a table knife.
A few pegmatites contain scattered red-brown crystals of garnet ranging in size from that of a BB shot to a small marble; a few in Garnet Canyon and Glacier Gulch are larger than baseballs (fig. 25). The garnets are fractured and many are partly altered to chlorite (a dull-green micaceous mineral) so they are of no value as gems.
Pegmatite dikes (tabular bodies of rock that, while still molten, were forced along fractures in older rocks) commonly cut across granite dikes, but in many places the reverse is true. Some dikes are composed of layers of pegmatite alternating with layers of granite (fig. 26), showing that the pegmatite and granite are nearly contemporaneous. Prof. Bruno Giletti and his coworkers at Brown University, using the rubidium-strontium radioactive clock, determined that the granite and pegmatite in the Teton Range are about 2.5 billion years old.
Even the most casual visitor to the Teton Range notices the remarkable black band that extends down the east face of Mount Moran (figs. 27 and 28) from the summit and disappears into the trees north of Leigh Lake. This is the outcropping edge of a steeply inclined dike composed of diabase, a nearly black igneous rock very similar to basalt. Thinner diabase dikes are visible on the east face of Middle Teton, on the south side of the Grand Teton, and in several other places in the range (see geologic map inside back cover).
The diabase is a heavy dark-greenish-gray to black rock that turns rust brown on faces that have been exposed to the weather. It is studded with small lath-shaped crystals of feldspar that are greenish gray in the fresh rock and milk white on weathered surfaces.
The black dikes formed from molten rock that welled up into nearly vertical fissures in the older Precambrian rocks. Toward the edges of the dikes the feldspar laths in the diabase become smaller and smaller (fig. 29), indicating that the wall rocks were relatively cool when the magma or melted rock was intruded. Rapid chilling at the edges prevented growth of large crystals. In many places hot solutions from the dike permeated the wall rocks, staining them rosy red.
The black dike on Mount Moran is about 150 feet thick near the summit of the peak. This dike has been traced westward for more than 7 miles. Where it passes out of the park south of Green Lakes Mountain it is 100 feet thick. The amount of molten material needed to form the exposed segment of this single dike could fill Jenny Lake three times over. The other dikes are thinner and not as long: the dike on Middle Teton is 20 to 40 feet thick, and the dike on Grand Teton is 40 to 60 feet thick.
The black dikes must be the youngest of the Precambrian units because they cut across all other Precambrian rocks. The dikes must have been intruded before the beginning of Cambrian deposition inasmuch as they do not cut the oldest Cambrian beds. Gneiss adjacent to the dike on Mount Moran contains biotite that was heated and altered about 1.3 billion years ago according to Professor Giletti. The alteration is believed to have occurred when the dike was emplaced; therefore this and similar dikes elsewhere in the range are probably about 1.3 billion years old.
At about the same time as the dikes were being intruded in the Tetons, many thousands of feet of sedimentary rocks, chiefly sandstone, were deposited in western Montana, 200 miles northwest of Grand Teton National Park. The sandstone was later recrystallized and recemented and became a very dense hard rock called quartzite. Similar quartzite, possibly part of the same deposit, was laid down west of the north end of the Teton Range, within the area now called the Snake River downwarp (fig. 1).
The visitor who hikes or camps anywhere on the floor of Jackson Hole becomes painfully aware of the thousands upon thousands of remarkably rounded hard quartzite boulders. He wonders where they came from because nowhere in the adjacent mountains is this rock type exposed. The answer is that the quartzites were derived from a long-vanished uplift (figs. 42 and 46), carried eastward by powerful rivers past the north end of the Teton Range, and then were deposited in a vast sheet of gravel that covered much of Jackson Hole 60 to 80 million years ago. Since then, these virtually indestructible boulders have been reworked many times by streams and ice, yet still retain the characteristics of the original ancient sediments.
A backward glance
So far we have seen that the Precambrian basement exposed in the Teton Range contains a complex array of rocks of diverse origins and various ages. Before passing on to the younger rocks, reference to our yardstick may help to place the Precambrian events in their proper perspective.
In all of Precambrian time, which encompasses more than 85 percent of the history of the earth (31 of the 36 inches of our yardstick), only two events are dated in the Teton Range: the intrusion of granite and pegmatite about 2.5 billion years ago, and the emplacement of the black dikes about 1.3 billion years ago. These dates are indicated by heavy arrows on the time scale (fig. 30). The ancient gneisses and schists were formed sometime before 2.5 billion years ago, and probably are no older than 3.5 billion years, the age of the oldest rocks dated anywhere in the world.
The close of the Precambrianend of the beginning
More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocksa longer period of time than has elapsed since the beginning of the Paleozic Era. During this enormous interval the Precambrian rocks were uplifted, exposed to erosion, and gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly floundered and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years. It is to the sediments deposited in these seas that we turn to read the next chapter in the geologic story of the Teton Range.
Last Updated: 19-Jan-2007